1. Field of the Invention
Implementations of various technologies described herein generally relate to seismic acquisition.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
A seismic survey typically includes a plurality of seismic sources and an acquisition system consisting of a central system and a plurality of deployed sensors configured to measure and record signals reflected from sub-surface layers as a result of the force exerted on the earth by the seismic sources.
In a vibroseis seismic survey, one or more vibrator units are typically used as the seismic source. Each vibrator unit may include a vehicle mounted actuator, capable of exerting force on the ground, typically in the form of a sweep of frequencies. Each vibrator unit may be controlled by a vibrator control system, which is configured to create a specified reference sweep and ensure that force exerted on the ground by the actuator follows the reference sweep as closely as possible. The coordination of multiple vibrator units and their corresponding vibrator control systems is typically accomplished by a recorder vibrator control system found in the acquisition system. The recorder vibrator control system sends synchronization signals via radio to the individual vibrator control systems that define the time the vibrator unit will start its sweep.
Several disadvantages exist with current synchronization technologies, which typically depend on setting various measured delay values to compensate for radio transmission times and other factors. The initial measurement of these delays and confirmation of system timing can be a time consuming process.
In addition, post acquisition checks may be performed to measure the accuracy of internal oscillators within vibrator control systems and gross errors in radio delays or start times. Checks during acquisition may also be performed to measure the accuracy of synchronization for specific sweep starts.
Checks performed periodically or during acquisition may be known as similarity checks. Two types of similarity checks are commonly used. The first, a radio similarity check, may take place several times daily for each vibrator in use during a normal acquisition cycle. In this method, a sweep signal from a vibrator unit's vibrator control system is passed in real time by analog modulation over a radio channel to the recorder vibrator control system, where it can be compared to a special reference signal, which was previously adjusted to be time consistent with the received signal. The radio similarity method can provide a check on the timing of the vibrator's sweep relative to that reference, but assumes all previously measured delays and adjustments are correct. The radio similarity method can also provide information about the quality of the vibrator sweep, such as its relative phase, its force and its level of distortion. While the data is acquired during normal production, the analysis of the data for all vibrators in use may be time consuming.
The second method, known as a wireline or hardwire similarity, is typically not performed during a normal acquisition, but during a test schedule or following maintenance on the vibrators, which may be time consuming. In the second method, one or more vibrator units may be positioned so that sweep signals can be acquired by direct connection to the acquisition system or its field units. Like the first method, analysis of the data may also be a time consuming process.